The process of bending sheets in a plurality of sub-steps is already known. For example, the edges of composite sheets are bent round in sub-steps during folding by approximately 180°. In the conventional processes, the edge of the sheet is initially edge-bent to approximately 90° and then pre-folded to 120° to 130°. Subsequently, the seam is produced by bending the pre-folded area round to approximately 180°. Sheets can be joined one below the other, for example, by means of the seam. On the other hand, less sharp-edged component edges can also be produced by folding.
Furthermore, a method for bending composite sheets, in which a pivotable bending die is used and the bending punch has a recessed notch in the bending edge, is known from the German published patent application DE 10 2010 016 914 A1. It has become apparent that in the conventional production process, in which the bending edge is initially edge-bent to 90°, pre-folded and then folded into the finished form, composite sheets are prone to exhibiting cracks, so that the seam is defective. The method known from the mentioned published patent application for producing a seam of a composite sheet results in a delamination in the area of the bending edge. When the seam is being produced, then in the area of the bending edge under some circumstances a crack also occurs in the outer metallic layer or the bending behaviour of the metallic layer is uncontrolled.
Taking this situation as the starting point, the object forming the basis of the invention is to make a method for bending a composite sheet available, in which cracks in the metallic layers of the composite sheet can be prevented and, at the same time, in which a seam can easily be provided.